Glioblastoma multiforme (GBM) represents the most common form of primary brain cancer with a two- year survival rate of ~ 26% following the best therapy. The capacity to advance the course of this disease therefore very much depends on our abilities to design and test novel therapies. Our group has been focused on the development of conditionally replicative adenoviruses (CRAds). CRAds are engineered to selectively replicate within and kill tumor cells through the use of transductional modifications t enhance viral infectivity and tumor-selective promoter elements that transcriptionally restrict expression of genes essential for viral replication. The potential success of adenoviral-based virotherapy has, however, been limited in practice. Preclinical work suggests that oncolytic viruses promote immune responses, which outweigh direct oncolysis in mediating anti-tumor efficacy. In fact, the major challenge of oncolytics lies in the difficult task of understanding ho to stimulate profitable anti-tumor immunity in the context of preexisting antiviral immunity. Present clinical data support preclinical observations that anti-tumor immune responses are important to long- term oncolytic virotherapy efficacy. Consequently, elucidating the mechanisms which drive the balance between anti-viral immunity vs. anti-tumor immunity in the central nervous system (CNS) will be key in mediating successful oncolytic virotherapy against GBM. As we look into the next phase of our project, we face several important questions with fundamental implications for the field of brain tumor virotherapy: (1) is it desirable to overcome anti-viral immunity? (2) is anti-tumor immunity more important than oncolysis? and (3) can one quantify this and consistently manipulate the host immune response against intracranial tumors? These three problems serve as the basis for our renewal application and for our three specific aims that together test the central hypothesis Regulatory T cell inhibition promotes vira oncolysis and long-term anti-tumor response in the context of oncolytic virotherapy of GBM.